Although hyperactivity is often treated as if it were a single phenomenon, various kinds of alterations in motor movements are involved in hyperactivity resulting from damage to different brain areas. Two types of drug-induced hyperactivity (amphetamine and morphine) and three types of brain lesions (gestational x-irradiation, neonatal exposure to carbon monoxide, and stereotaxic lesions in neonatal and adult rats) are being studied. Comparisons to date have shown that many general similarities exist in the type of hyperactivity: durations of many behavior acts are shortened and frequencies of locomotor acts are increased. Other details of hyperactive behavior vary among the different types of brain alteration. Even when the behavioral changes are similar, marked differences in the neurons in several brain areas have been found. The morphological changes in the caudate neuron, using the Golgi stain, are under investigation.